Process for producing low-carbon chromium steels



' Patented Mar. 27,1951

PROCESS FOR PRODUCING LOW-CARBON CHROMIUM STEELS Donald Cleve Hilty, Niagara Falls, N. Y., assignor to Union Carbide and Carbon Corporation, a corporation of New York No Drawing. Application November 14, 1949, Serial No. 127,257

The invention relates to the manufacture of chromium-containing steels and in particular to such steels containing to 25% chromium and less than 0.20% carbon.

conventionally, in the manufacture of chromium-containing steels an electric arc furnace is used. Into such furnace is placed a charge containing chromium-containing scrap, plain carbon steel scrap and, if the steel is to contain nickel, nickel-containing material, with or without the addition of slag-forming material. The charge is then melted to form a liquid bath. Be cause of the difiiculty of segregating scrap by exact composition with respect to its carbon content, and also because of the limited availability of very low-carbon steel scrap, the carbon content of the bath at melt-down is variable, and usually contains more carbon than is desired in the final product. The excess carbon is removed by oxidation. Any of a variety of oxidants, such as iron oxde, chrome ore, or nickel oxide, may be employed. More recently, it has been the custom to inject gaseous oxygen or air directly into the molten charge to remove carbon. The gaseous oxidation technique usually results in a large temperature rise of the bath as contrasted with the chilling efiect of solid oxidants.

During the oxidation period, not only carbon but also silicon, manganese, iron, and chromium are oxidized in attaining the desired carbon level. In order to conserve that portion of the chromium which has been oxidized and is present in the slag, reducing agents are added to the slag bath to re duce the oxidized chromium into the steel bath. This reducing operation is often continued until the chromium oxide content of the slag has been descreased to a figure as low as 3%. The slag is then removed from the furnace, and additional slag-forming materials are fed. Usually, this second slag is non-oxidizing or reducing in character, although it may be mildly oxidizing. Additional chromium in the form of low-carbon ferrochromium is added through the slag to the bath untilgthe desired chromium content of the finished steel is reached. The bath is then finally deoxidized and minor alloying additions made, after which the furnace is tapped and the steel cast.

It is apparent from the above that recovery of chromium from the slag influences both the time necessary to make the heat and the final cost of the steel. In order to reduce oxidized chromium from the slag, relatively expensive silicon-bearing or aluminum-bearing alloys are employed, and the time consumed in the reducing operation 5 Claims. (01. 75-1305) is a direct function of the amount of chromium contained in the slag bath at the end of the oxidizing period. Steelmakers have long recognized that under existing practices the partition of the original chromium fed in the charge between the steel bath and the slag bath during the oxidation period is beyond their control.

It has been suggested that the amount of chromium oxidized from the steel bath could be minimized by feeding chromium oxides to the charge, for example, in the form of chrome ore. This procedure, however, is not a satisfactory answer for the problem, since the chromium oxide in the slag, regardless of its original source, must be reduced in order to recover it in the final product. Accordingly, it has been the general practice among steelmakers to keep the carbon content of the steel bath at melt-down as low as possible, since it was supposed and generally accepted that the loss of chromium from the bath was directly proportional to the amount of carbon that had to be removed.

- Among the governin factors involved in the oxidation of carbon from a chromium-containing steel bath are the carbon and chromium contents and temperature of the molten steel, At equilibrium, it is only possible to retain a certain percentage of chromium in the steel for any designated carbon content, temperature being constant. Similarly, with the same carbon content, the amount of chromium that can be retained increases with increasing temperature; and again, with constant temperature, the amount of chromium that may be retained increases with increasing carbon content, as described in my publication, Relation between Chromium and Carbon in Chromium Steel Refining, in Transactions American Institute of Mining & Metallurgical Engineers, volume 185, 1949, pages 91-95.

' I have discovered, however, that under the conditions prevailing in steelmaking furnaces, this limiting equilibrium is approached much more easily and with much greater rapidity from the direction of excess carbon than from the direction of excess chromium. That is, with increasing temperature, and under oxidizing conditions, the oxidation of carbon proceeds at a high rate and is easily effected, while the reversion of chromium an oxidized form to metallic chromium dissolved in the steel bath, as

demanded by the influence of increasing tempera ture on the equilibrium, occurs only very slowly and lags far behind the normal rate of temperature rise in practical steelmaking operations.

The practical significance of this discovery is as follows:

In any steelmaking process, the temperature of the bath at melt-down is relatively low. In fact, the temperature of the bath during the melting period never rises much above the melting point of steel until the charge is completely molten. Thus, the oxidizing period begins at a rather low bath temperature.

If, therefore, the chromium Having made the carbon addition, the charge is melted in the usual way. When the bath is completely molten, oxidation of the carbon is begun. Although any of the oxidizing methods known to the art may be employed, I prefer the injection of gaseous oxygen because of its'greater speed and flexibility in attaining the high temperatures necessary for retaining the maximum amount of chromium in the bath. When the content of the change is high with respect to the carbon content has been lowered to the desired carbon content, as has heretofore been customary level, the oxidation is discontinued and the oxides in practice, a considerable portion of the chro of the slag are reduced by the addition of an mium is oxidized into the slag during the early appropriate reducing agent. Following the restages of the oxidizing period. During the oxiduction step the heat is finished and tapped in dizing period the temperature may rise quite the conventional manner. As has been menrapidly, particularly if the oxidation is carried tioned, however, the amount of chromium oxiout by means of the injection of gaseous oxygen dized during the oxidation of the carbon is much into the bath. However, because the reversion of less than in conventional practice, so that the the oxidized chromium from the slag back into amount of reducing agents needed and the time the bath occurs at an extremely slow rate, equirequired to accomplish reduction of the slag are librium is not attained at the end of the oxidizing also considerably less.

period, and the chromium content of the bath The advantages of this process are illustrated at that time may be much less than that perby the following examples or" results obtained mitted by the carbon content and the temperafrom 2000 pound arc furnace heats:

Composition of Charge gg g gfi After Pounds of Per Cent of Pounds of Lime to Time for Heat Chro- Chrooir r ii ii n Hittite st ni e- 1523 mium, P06111112? fgg mium, gfififf Oxidized Slag Basicity tion,min.

Cent 55; mium Cent 5 mium 1 Reducuon I 0. 27 11.1 228 0. 05 s. 1 155 32. o 36. 5 55 47 III 0.78 10.6 216 0. 05 9.1 176 18.5 22.8 34 so 1 Calculated on basis of change in bath weight during oxidizing period.

ture. On the other hand, if the carbon content of the charge is high with respect to the chromium, chromium is not oxidized until the carbon content of the bath has been reduced to the equilibrium level, and, therefore, the amount of chromium that is retained in the bath is as high is con istent with the equilibrium at the temperature attained when the carbon oxidation is completed.

Based upon this discovery, I have developed an improved process for producing chromium steels. In this new process I deliberately add to a chromium steel heat considerably more carbon than would normall be introduced by the materials of the charge. This procedure materially decreases the amount of total chromium oxidized, consequently lessening the amount of reducing agent required to reclaim such oxidized chromium from the slag, and, in general, shortening the heat-making time, all of which result in substantial savings in the cost of materials and labor involved.

Essentially, my process consists of the intentional addition of carbon in excess of the amount that would be in equilibrium with the chromium content of the bath at the low temperature prevailing during the early stages of a heat. Good results can be obtained, for example, with additions of one pound or more of carbon for every 20 pounds of chromium in the charge. The addition may be made at any time prior to the start of direct oxidation of the bath, but to obtain the best results, I prefer to include the addition in the original charge or to make it be fore much of the charge has become molten. The carbon may be added in any conveniently available form, such as pig iron, high-carbon ferrochromium, cast iron scrap, or elemental carbon.

In the above table, Heat I was made by conventional practice wherein excess carbon was not added to the charge. In Heats II and III, extra carbon was added in an amount in excess of that which would be in equilibrium with the chromium content of the bath at the low temperatures prevailing during melt-down. All heats were oxidized to the same extent as indicated by the carbon contents of the bath after oxidation. The decreased oxidation of chromium and resultant savings in materials and time brought about by the addition of carbon to the charges for Heats II and III made according to the invention are manifest from the table.

What is claimed is: a

1. In the manufacture of chromiiun-containing steel containing less than 0.20% carbon including the steps of preparing and melting a charge of materials to form a bath of molten steel containing carbon and chromium, oxidizing such bath to remove carbon, reducing a portion of the metal oxides formed during said oxidizing step, deoxidizing said bath, and tapping, the improvement which comprises adding carbon to said charge prior to the start of direct oxidation of the bath in an amount in excess of that which would be in equilibrium with the chromium content of the charge at the temperatures prevailing during the melt-down period thereby decreasing the amount of chromium oxidized in said oxidizing step.

2. In the manufacture of chromiinn-containing steel containing less than 0.20% carbon including thesteps or preparing and melting :a charge of materials to form a vbath'loi' molten steel containing carbon and chromium, .oxi'diz'-; ing such bath to remove carbon, deoxidizing said bath, and tapping, the improvement which com prises adding carbon to said charge prior to the start of direct oxidation of the bath in an amount in excess of that which would be in equilibrium with the chromium content of the charge at the temperatures prevailing during the melt-down period thereby decreasing the amount of chromium oxidized in said oxidizing step.

3. The improvement claimed in claim 1, wherein at least one pound of carbon is contained in said charge for each 20 pounds of chromium therein.

4. The improvement claimed in claim 1, wherein at least a portion of the excess carbon addition is made in the form of high-carbon ferrous material.

5. The improvement claimed in claim 1, wherein at least a portion of the excess carbon addition is made in the form of elemental carbon.

DONALD CLEVE HILTY.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,410,749 Hadfield Mar. 28, 1922 1 1,641,326 Farnsworth Sept. 6, 1927 

1. IN THE MANUFACTURE OF CHROMIUM-CONTAINING STEEL CONTAINING LESS THAN 0.20% CARBON INCLUDING THE STEPS OF PREPARING AND MELTING A CHARGE OF MATERIALS TO FORM A BATH OF MOLTEN STEEL CONTAINING CARBON AND CHROMIUM, OXIDIZING SUCH BATH TO REMOVE CARBON, REDUCING A PORTION OF THE METAL OXIDES FORMED DURING SAID OXIDIZING STEP, DEOXIDIZING SAID BATH, AND TAPPING, THE IMPROVEMENT WHICH COMPRISES ADDING CARBON TO SAID CHARGE PRIOR TO THE START OF DIRECT OXIDATION OF THE BATH IN AN AMOUNT IN EXCESS OF THAT WHICH WOULD BE IN EQUILIBRIUM WITH THE CHROMIUM CONTENT OF THE CHARGE AT THE TEMPERATURES PREVAILING DURING THE MELT-DOWN PERIOD THEREBY DECREASING THE AMOUNT OF CHROMIUM OXIDIZED IN SAID OXIDIZING STEP. 